Cardiac cycle – pressure, heart valves, ECG and control of heart rate
BiologyHuman & Animal PhysiologyAges 16–17
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Sign in to playA four-chamber heart beats in slow motion: the sinoatrial node (SAN) fires, the wave spreads over the atria, is delayed at the atrioventricular node (AVN) and runs down the bundle of His to the Purkinje fibers. Pressure–time graphs for the left atrium, left ventricle and aorta show the atrioventricular and semilunar valves opening and closing exactly when the pressure gradient reverses, with ventricular volume and an ECG (P, QRS, T) aligned below. Change blood CO₂ (chemoreceptors), arteriole resistance and blood volume (baroreceptors) to see the medulla adjust heart rate through the sympathetic and vagus nerves, and read stroke volume and cardiac output = stroke volume × heart rate.
Lesson: Cardiac cycle, myogenic heartbeat, ECG and nervous control of heart rate
What it shows
The heart is myogenic: the sinoatrial node sets the rhythm, the atrioventricular node delays the impulse so the atria empty first, and the bundle of His and Purkinje fibers make the ventricles contract from the apex upward. Valves open and close passively whenever the pressure on one side becomes higher than on the other, as the pressure–time graph shows for the left atrium, left ventricle and aorta. Chemoreceptors and baroreceptors send impulses to the medulla, which changes heart rate through sympathetic and parasympathetic (vagus) nerves; cardiac output equals stroke volume times heart rate.
How to use
Watch one cycle at a slow Playback speed, then press Pause or Step 0.02 s and match the numbered valve events to the crossing pressure curves and to the ECG. Next raise Blood CO₂, lower Arteriole resistance or Blood volume, or press Exercise or Blood loss, and record heart rate, stroke volume and cardiac output in a table. Tick Cut the nerves to the heart to reveal the myogenic rate of the SAN.
Parameters you can change
- Blood CO₂ (chemoreceptors) 4–8 kPa
- Arteriole resistance (baroreceptors) 50–200 %
- Blood volume 70–120 %
- Cut the nerves to the heart (transplanted heart)
- Playback speed 0.1–1 × real time
- Pressure unit kPa, mmHg
Questions to explore
- Why does the atrioventricular valve close at the start of ventricular systole, before any blood leaves the ventricle?
- Why is the impulse delayed at the atrioventricular node, and where does this delay appear on the ECG?
- How does a fall in blood volume change heart rate, stroke volume and cardiac output, and why?
